posted on 1996-07-23, 00:00authored byGuillaume Desurmont, Rita Klein, Stefan Uhlenbrock, Eric Laloë, Laurent Deloux, Dean M. Giolando, Yong Wah Kim, Schubert Pereira, Morris Srebnik
Hydrozirconation of 1-alkynyl pinacolboronates, 1, with
HZrCp2Cl provides gem-borazirconocenes 2. The latter when treated with CuBr gives the
homocoupled (1E,3E)-2,3-dibora-1,3-butadienes, 3, in good yield (62−67%). The
reaction works even for hindered 2 (R =
t-Bu). Structure 3 was assigned on the basis
of NMR. Suzuki−Miyaura coupling of 3a (R
= n-Bu) with PhI in the presence of CsF leads to the
replacement of both boron groups by
phenyl and hydrogen to give 6 in 76% yield.
Zirconocene-mediated coupling of 1 leads
to
diastereomeric products:
(1E,3E)-1,3-dibora-1,3-butadienes, 4,
in 7−26% isolated yields and
(1E,3E)-1,4-dibora-1,3-butadienes, 5,
in 17−34% isolated yields. The two isomers can be
separated by selective precipitation of 5 from the reaction
mixture in pentane (−20 °C)
followed by silica gel chromatography to give pure 4.
The reaction does not work when R in
1 is the t-Bu group. Assignments of
structures for 4 and 5 were done on the basis of
1D and
2D NMR experiments. In addition, a single-crystal X-ray analysis
of 5a showed it to be a
highly planar and linearly oriented molecule. Suzuki−Miyaura
coupling of 4a proceeded to
replace the terminal boron group exclusively, while the internal boron
group of 4a remained
intact.